Zero controllability in discrete-time structured systems

In this paper we consider complex dynamical networks modeled by means of state space systems running in discrete time. We assume that the dependency structure of the variables within the (nonlinear) network equations is known and use directed graphs to represent this structure. The dependency structure also appears in the equations of a linearization of the network. In order for such a linearization to be a good approximation of the original network, its state should stay as close as possible to the point of linearization. In this paper, we investigate how the latter can be achieved by an appropriate selection of states as driver nodes, so that through these driver nodes the whole state of the network can be steered to the point of linearization. We present conditions in graph terms for this to be possible and deriver an algorithm for the associate driver node selection. By means of a simple reasoning, we show that finding such a selection of smallest size comes down to solving a minimal cover problem, which is known to be an NP-hard problem.